In many aircraft, main propulsion engines not only provide propulsion for the aircraft, but may also be used to drive various other rotating components such as, for example, generators, compressors, and pumps, to thereby supply electrical and/or pneumatic power. However, when an aircraft is on the ground, its main engines may not be operating. Moreover, in some instances the main propulsion engines may not be capable of supplying the power needed for propulsion as well as the power to drive these other rotating components. Thus, many aircraft include an auxiliary power unit (APU) to supplement the main propulsion engines in providing electrical and/or pneumatic power. An APU may also be used to start the propulsion engines.
An APU is typically a gas turbine engine that includes a combustion section, a power turbine section, and a compressor section. During operation of the APU, the compressor section draws in and compresses ambient air and supplies the air to the combustion section. Fuel is injected into the compressed air within the combustion section to produce the high-energy combusted air to the power turbine section. The power turbine section rotates to drive a generator for supplying electrical power, via a main shaft, and to drive its own compressor section and/or an external load compressor.
Many APUs include an exhaust gas temperature (EGT) sensor. The EGT sensor, as its nomenclature denotes, senses the temperature of the exhaust gas discharged from the APU. If the EGT sensor becomes inoperable or otherwise unavailable, the APU may be prevented from being started up. This is done to prevent the APU from experiencing undesirably high temperatures during start-up, which can concomitantly reduce the overall life of the APU. For some aircraft operating scenarios, it would be desirable to allow an APU to be started up even though the EGT sensor may be inoperable or otherwise unavailable.
Hence, there is a need for a system and method of starting an APU when its associated EGT sensor is inoperable or otherwise unavailable while preventing the APU from experiencing undesirably high temperatures. The present invention addresses at least this need.